This invention relates to an electrosurgical generator for delivering an electrosurgical current particularly but not exclusively in intracavitary endoscopic electrosurgery. The invention also relates to an electrosurgical system comprising the combination of a generator and an electrode assembly. The term xe2x80x9cintracavitaryxe2x80x9d is used in this specification to denote electrosurgery in which living tissue is treated by least invasive surgical access to a body cavity. This may involve xe2x80x9cunderwater electrosurgeryxe2x80x9d, a term denoting that the surgery is performed using an electrosurgical instrument with a treatment electrode or electrodes immersed in liquid at the operation site. The invention has particular application in the fields of urology, hysteroscopy and arthroscopy.
Intracavitary endoscopic electrosurgery is useful for treating tissue in anatomical or surgically created cavities of the body which can be accessed by methods involving minimal trauma to the patient, be this through a natural body passage or one created artificially, The cavity is distended to provide space for gaining access to the operation site to improve visualisation and to allow for manipulation of instruments. In low volume body cavities, particularly where it is desirable to distend the cavity under higher pressure, liquid rather than gas is more commonly used due to better optical characteristics and because it washes blood away from the operative site. Conventionally, a non-electrolyte solution such as glycine is used as the fluid distension medium when electrosurgery is being used, glycine being electrically non-conductive.
The limited surgical access encountered during intracavitary endoscopic procedures makes the removal of tissue pieces derived from a typical electrosurgical loop cutting electrode both difficult and time consuming. Vaporisation of tissue whereby the tissue is reduced to smoke and water vapour is a preferable technique in these situations, rather than the piecemeal removal of tissue. The products of vaporisation may be removed following dissolution within a liquid irrigating medium.
With regard to underwater endoscopic electrosurgery, the applicants have found that it is possible to use a conductive liquid medium such as normal saline in place of glycine. Normal saline is the preferred distension medium in underwater endoscopic surgery when electrosurgery is not contemplated or a non-electrical tissue effect such as laser treatment is being used. Although normal saline (0.9%w/v; 150 mmol/l) has an electrical conductivity somewhat greater than that of most body tissue, it has the advantage that displacement by absorption or extravasation from the operative site produces little physiological effect and the so-called water intoxication effects of glycine arc avoided.
Effective electrosurgical treatment of tissue which is totally immersed in liquid at the application site is difficult to achieve because the heat generated by the flow of electrical currents in both the tissue being treated and surrounding conductive liquid tends to cause boiling of the liquid. The operating electrode is intermittently surrounded by water vapour rather than liquid, with consequent large variations in the electrical impedance of the load presented to the generator supplying the electrosurgical power to the electrode. Whilst this variation is mitigated by use of a non-conductive liquid, it cannot be eliminated entirely due to the release of body fluids at the operative site which elevates the electrical conductance of the liquid. Changes in tissue type also alter the load impedance. These effects result in difficulty in controlling the electrosurgical output to produce consistent effects on the tissue being treated. As a result, high powers are commonly employed to overcome this performance variation.
According to a first aspect of this invention, an electrosurgical generator for supplying radio frequency power to an electrical instrument, comprises a radio frequency output stage having at least a pair of electrosurgical output connections for the delivery of radio frequency power to the instrument, and control circuitry operable to limit the radio frequency peak output voltage developed across the output connections to at least first and second predetermined threshold values and, in a blend mode of the generator, to alternate constantly between said first and second threshold values. The output stage preferably comprises a resonant output circuit coupled to the output connections and a switching device coupled to the resonant output circuit, and wherein the control circuitry is operable to actuate the switching device to reduce the delivered radio frequency power. The switching device is preferably connected between the resonant output circuit and one of a pair of supply rails of the power supply means, and connected so as to switch current repeatedly through the resonant output circuit at its resonant frequency. In order to cause a control overshoot; in terms of the degree to which the delivered power is reduced when the output voltage reaches the predetermined threshold, the control circuitry is so arranged and coupled to the switching device that it is capable of reducing the xe2x80x9conxe2x80x9d time of the switching device during individual radio frequency switching cycles sufficiently rapidly to cause a 50% reduction in delivered output power within 100 xcexcs of the predetermined threshold having been reached. This allows surgery to be performed in a conductive fluid field, in particular in a saline solution. Large and rapid changes in load impedance can occur substantially without causing unwanted electrosurgical effects. For example, when it is desired to produce electrosurgical desiccation, any increase in impedance due to vaporisation of surrounding saline in the region of an electrode of the instrument which might otherwise lead to unwanted arcing at the required power level for effective desiccation can be largely prevented. When electrosurgical tissue cutting or tissue vaporisation is required, output voltage limitation can be used to prevent electrode burning and/or excessive tissue vaporisation. In the blended mode, the above two states are used alternately, wherein a pocket of vapour continually forms and collapses in rapid succession.
The control circuitry may include a control line feeding a first power reduction control signal to the radio frequency output stage. The output stage, which may he a radio frequency power oscillator, typically has as the oscillating element a radio frequency power device, and in the preferred embodiment, the control circuitry is arranged such that at least a 50% reduction in output power is brought about in a period of less than 20 xcexcs after the output voltage reaches the predetermined threshold by reducing the period of conduction of the device during individual cycles of the radio frequency output signal. Such alteration in the period of conduction is advantageously achieved independently of any variation in supply voltage to the radio frequency power device. In practice, the reduction in Output power is brought about using a single control variable, i.e. the peak output voltage or peak-to-peak output voltage, independently of supply voltage and independently of the delivered output power which varies according to the load impedance and the supply voltage. Thus, triggering of a power reduction occurs at the same preset output voltage threshold but at different output power and load impedance values, according to circumstances.
As an adjunct to direct control of the radio frequency output stage, the means for causing a reduction in output power may include a further control line which is coupled to the power supply means, the control circuitry being arranged such that a second power reduction signal is fed to the power supply means to effect a reduction in the average power supply voltage supplied to the output stage. Typically, the rate of reduction of power due to lowering of the power supply voltage is comparatively slow, but the combination of two means of control can produce a larger range of available output power levels.
In the preferred generator the control circuitry has a first output coupled to a radio frequency power device in the output stage to reduce the radio frequency duty cycle thereof and a second output coupled to the power supply to effect a reduction in the average power supply voltage supplied to the output stage, the said reductions occurring in response to the sensing signal reaching a respective predetermined threshold value, depending on the mode of treatment required.
In the case of the power supply being a switched mode power supply having output smoothing components, the supply circuit may be arranged such that the second power reduction control signal has the effect of disabling the supply circuit, e.g. by gating the pulsed output. Accordingly, a high-speed control response is obtained with the supply voltage falling relatively slowly after the initial step power reduction to enable the radio frequency duty cycle of the power device to be increased again, thereby allowing further high-speed power reductions if necessary.
The technique of directly controlling the radio frequency output stage can be performed by repeatedly producing, firstly, a rapid reduction in the cycle-by-cycle conduction period of the power device from a peak level to a trough level when the respective output threshold is reached, followed by, secondly, a progressive increase in the conduction period until the conduction period again reaches its peak level, the radio frequency output voltage being monitored during the progressive increase. This rapid reduction and progressive increase sequence may be repeated until the peak conduction period level can be reached without the output voltage exceeding the respective output threshold due to the supply voltage from the switched mode power supply having fallen sufficiently since it was disabled. Re-enabling of the supply circuit typically occurs after a delay, and conveniently at the end of the first switched mode switching cycle in which the output voltage has not reached the threshold for the whole of the switching cycle. It will be appreciated that, during the blended mode of operation, the repeated reduction and restoration of the cycle-by-cycle conduction period of the power device typically occurs many times during each period of tissue coagulation or vaporisation. In other words, it occurs at a much faster rate than the rate of alternation between states.
The output stage preferably includes an output resonant circuit having a Q which is sufficiently high to remove switching noise from the switching device or devices of the stage without unduly slowing the response to the output voltage reaching the predetermined threshold. Typically, the Q is sufficient to achieve a crest factor below 1.5, the crest factor being the ratio of the peak and r.m.s. values of the output voltage waveform.
The generator may have an output impedance in the range of from 100 ohms to 250 ohms, and preferably between 130 and 190 ohms. Such a generator has its radio frequency output stage operable to produce a CW (continuous wave) output, i.e. with a 100% duty cycle or without on/off pulse width modulation at a frequency lower than the r.f. oscillation frequency. In effect, the output stage may operate as an open loop stage with a power/load impedance characteristic having a peak (preferably a single peak) at about 150 to 160 ohms and with the curve decreasing continuously with decreasing impedance below the peak and increasing impedance above the peak.
Another view of the preferred generator is that of apparatus for supplying radio frequency power to an electrosurgical instrument for operation in an electrically conductive fluid medium, the generator comprising a radio frequency output stage having a radio frequency power device and at least a pair of electrosurgical output connections for the delivery of radio frequency power to electrodes, power supply means coupled to the output stage, and control circuitry including sensing means for deriving a sensing signal representative of the radio frequency output voltage developed across the output connections and means responsive to the sensing signal for causing a reduction in delivered output power when the sensing signal is indicative of a predetermined output voltage threshold having been reached, wherein the control circuitry is arranged such that the reduction in output power is effected by reducing the period of conduction of the device during individual cycles of radio frequency oscillation, preferably independently of the supply voltage to the device.
The generator has at least a pair of electrosurgical output connections for the delivery of radio frequency power to the instrument, means coupled to the output stage for supplying power to the output stage, and control circuitry including sensing means for deriving a sensing signal representative of the radio frequency output voltage developed across the output connections and means responsive to the sensing signal for causing at least a 50% reduction in delivered output power when the sensing signal is indicative of a predetermined output voltage threshold having been reached, the said reduction being effected within a period of 20 xcexcs or less.
The invention also includes an electrosurgical system comprising an electrosurgical generator for generating radio frequency power and an electrosurgical instrument coupled to the generator, the instrument having an electrode structure for operation immersed in an electrically conductive liquid, wherein the system has a first mode of operation in which tissue is treated by the application of heat in the region of the electrode structure without forming an electrode-enveloping vapour pocket, and a second mode of operation in which the tissue is locally vaporised by energy transmitted from the electrode structure via an electrode enveloping vapour pocket, said first and second modes being defined by different respective electrical control parameters selected in the generator, and wherein the system has a third, blended mode of operation produced by constantly alternating between the first and second modes. The electrode structure may include a distal treatment electrode and a liquid contact electrode spaced proximally from the distal electrode, both electrodes being for use surrounded by the conductive liquid and each being connected to a respective one of the pair of output connections the control stage being operable to reduce the reduction time of the power device when the conductive liquid at the distal electrode is vaporised, The electrosurgical instrument may provide an electrode structure having juxtaposed first and second electrodes for immersion in the conductive liquid, the first and second electrodes respectively forming a tissue treatment electrode at an extreme distal end of the instrument and a return electrode proximally spaced from the tissue contact electrode.
The preferred system is operable in a tissue desiccation mode, a tissue cutting or vaporisation mode and a blended mode, and comprises a generator for generating radio frequency power and an electrosurgical instrument coupled to the generator, the instrument having an electrode structure for operation immersed in a conductive liquid, wherein the generator includes a mode selection control and has power control circuitry for automatically adjusting the radio frequency power supplied to the electrode structure to limit the peak generator output voltage to a first value when the desiccation mode is selected and to at least one second value when the cutting or vaporisation mode is selected, the second value or values being higher than the first value, and to continually alternated first and second values when the blended mode is selected. The first and second values are advantageously in the ranges of from 150V to 200V, and from 250V to 600V respectively, these voltages being peak voltages.
From a method aspect, the invention provides a electrosurgical system comprising an electrosurgical radio frequency generator coupled to an electrode assembly having a treatment electrode; introducing the electrode assembly into a selected operation site with the treatment electrode contacting the tissue to be treated and with the tissue and the treatment electrode immersed in a conductive liquid; actuating the generator; and controlling the radio frequency power applied to the treatment electrode by the generator so as constantly to alternate between (a) a first treatment state in which the tissue to be treated is heated with the liquid adjacent the electrode maintained substantially at its boiling point without creating a vapour layer surrounding the electrode, and (b) a second treatment state in which said tissue is vaporised via a layer of vapour from the conductive liquid which is maintained around the electrode without overheating of the electrode, thereby to treat the tissue in a blended mode of operation in which the tissue is vaporised and neighboring tissue is coagulated. The radio frequency power supply to the electrode may be automatically adjusted by alternately limiting the output voltage to predetermined first and second voltage values, the first voltage value being used for desiccation and the second voltage value, which is higher than the first voltage value, being used for cutting or vaporisation to yield the required blended effect.
Alternatively, the output voltage may be modulated to produce a pulsed waveform in which bursts of radio frequency power are separated by periods of zero voltage output. Constant alternation between tissue desiccation and tissue vaporisation occurs, desiccation being obtained at the beginning of each burst prior to formation of a vapour pocket around the treatment electrode, and afterwards due to residual heat in the treatment electrode. Each time the vapour pocket forms, tissue vaporisation occurs.